Jupiter and the Moon to Show Off in Tuesday's Night Sky

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As darkness falls tomorrow (Dec. 6), check out the east-southeast
sky. You'll see a bright waxing gibbous moon, nearly 90
percent illuminated, and shining brilliantly to the lower right
of the moon will be a brilliant silvery "star" gleaming with a
steady glow.

The moon and Jupiter will be closest together during the
afternoon, and they will be slowly separating during the
overnight hours of tomorrow and Wednesday morning. During
the evening they'll be separated by about 5 or 6 degrees (for
comparison, your clenched fist held at arm's length measures
about 10 degrees and the moon moves across the sky at about its
own diameter each hour).

Jupiter remains a bold light high in the east-southeast at
nightfall. This week it doesn't set in the west
until around 3:30 a.m. local time.

Although brighter than any nighttime star, Jupiter blazes less
fiercely now than in previous months. One glance toward the
southwest before 6:30 p.m. local time will readily show that
Venus outshines it. Jupiter appears to be levitating in
front of the sparse stars of eastern Pisces, filling this region
of the sky with overbearing brightness.

What kind of telescopic observation can be made now? Almost
any kind. From mid-northern latitudes you could even watch
a full rotation of Jupiter, with the cloud features of every
longitude displayed, during a single nightlong vigil. And as
always, a fascinating dance of Jupiter's four large satellites
will await viewers on any night who watch with a small telescope
or even steadily held binoculars. [ Video:
Top 10 Amateur Telescopes ]

As darkness falls over the eastern United States tomorrow, you'll
see two of
Jupiter's moons — Ganymede and Europa — on one side of the
planet, while a third, Callisto, hovers by itself on the
other side.

As the evening progresses, notice how Ganymede and Europa will
gradually pull away from Jupiter, but then at 8:21 p.m. EST (5:21
p.m. PST), a fourth satellite (Io) will emerge from Jupiter's
shadow and l appear on the side of Jupiter occupied by
Callisto. Slowly, as tomorrow night wears on, Io will become
easier to see as it moves away from Jupiter and toward
Callisto.

Keep in mind that during this time, Jupiter's distance from
Earth is increasing, from 34.8 light-minutes now to 37.7
light-minutes by the end of December (a light-minute is the
distance light can travel in a minute — about 11 million miles,
or 18 million kilometers). The apparent size
of Jupiter's disk is therefore shrinking too; in fact, it
will be nearly 13 percent smaller by year's end compared
to when it was at opposition to the sun in late
October.

Nonetheless, Jupiter will continue to be a great target for those
who might acquire binoculars or a
telescope for the holidays over this upcoming winter season.

Joe Rao serves as an instructor and guest lecturer at New
York's Hayden Planetarium. He writes about astronomy for The New
York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester, New York.